The present invention relates generally to circuit testing and in particular the present invention relates to testing analog signals.
Integrated circuits typically include digital and analog circuitry. While circuitry provided in an integrated circuit is analog in nature, the analog circuits can be combined to provide a high level of system integration inside of a single integrated circuit chip. Analog signals are either voltage or current signals that vary over a specific range. Digital signals are either voltage or current signals that can have one of two discrete values, either a logic zero or logic one.
During manufacturing, it may be desired to test internal signals of the integrated circuit. To test signals in the integrated circuit, points in the circuit can be coupled to boundary scan circuitry that couples the signals to external connections. An analog boundary module can be used to connect analog signals from an integrated circuit core to an external pin. Likewise, a digital boundary module can be used to connect digital signals from the integrated circuit core to an external pin. Some integrated circuits include both analog and digital signals that need to be tested. These circuits are referred to as mixed signal circuits. Implementing boundary scan testing can result in the need for substantial dedicated external interconnect pins to route the test signal to test equipment.
Digital test equipment is used to test the digital signals and analog test equipment is used to test the analog signals. Mixed-signal testers are available, however, current mixed signal testers typically increase test time and are more expensive than traditional digital testers. IEEE test standard 1149.4 describes a standard for a mixed-signal test bus. This test bus can be coupled to a mixed-signal tester. Mixed-signal testers create extra expense and time to test mixed signal circuits, but are necessary to verify mixed-signal integrated circuits.
Further, problems in outputting analog signals from the integrated circuit can be experienced. That is, parasitic effects can change or corrupt the analog signals. In addition, the analog test equipment can have an influence on the circuitry being tested. Some analog signals are impossible to test externally due to these effects.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for circuitry and methods to test mixed signal circuits using common digital test equipment.
The above-mentioned problems with analog test circuits and other problems are addressed by the present invention and will be understood by reading and studying the following specification.
In one embodiment, an integrated circuit analog test cell comprises an analog input point to receive an analog signal, an analog reference signal input, an enable input, and a comparator coupled to the analog input point, analog reference signal input and an enable input. The comparator determines if the analog signal is within an analog range established in response to the analog reference signal and provides a digital output signal indicating a status of the determination. The integrated circuit can comprise a digital-to-analog converter circuit to generate the analog reference signal in response to a digital test signal.
In another embodiment, an integrated circuit analog test cell comprises a comparator circuit having first and second inputs, the second input is coupled to receive at least one analog reference signal, a plurality of analog input points to receive a plurality of analog signal, and a selection circuit coupled to the plurality of analog input points to selectively couple one of the analog signals to the first input of the comparator circuit. The comparator determines if an analog signal coupled to the first input is within an analog range established in response to the analog reference signal and provides a digital output signal indicating a status of the determination. Again, the integrated circuit can comprise a digital-to-analog converter circuit to generate the analog reference signal in response to a digital test signal.
A method of testing an integrated circuit comprises initiating a test mode, providing power to first and second analog test cells in response to the test mode, and providing an enable signal and a test reference signal to the first analog test cell. The method further comprises establishing a test range in response to the test reference signal, comparing an analog signal to the test range, and providing a digital output signal having a first state if the analog signal has a value within the test range. The digital output has a second state if the analog signal has a value that is outside the test range.